An oxygen probe of this kind is disclosed, for example, in U.S. Pat. No. 4,496,455. The oxygen probe described in this patent includes a measuring cell and a pump cell. The measuring cell is formed by an electrolyte having a measuring electrode and a reference electrode. The pump cell is configured by a further electrode pair on the same electrolyte. The measuring electrode is exposed to the exhaust gas of an internal combustion engine and the reference electrode is subjected to a reference atmosphere in a small volume within the exhaust-gas probe. This volume communicates with the exhaust gas or the ambient only via small bores via which an overpressure within the small volume can be reduced as may be required.
However, such a reference volume is not gas-tight with respect to the ambient or the exhaust gas because of these bores as well as a residual porosity of the electrolyte. A voltage is applied to the electrodes of the pump cell to maintain a stable reference atmosphere. This voltage is so dimensioned and directed that oxygen ions from the exhaust gas are pumped into the reference volume. Such an internal pumped reference atmosphere has the advantage with respect to an ambient air reference that an unwanted change of its composition is comparatively low. Such a change in composition can occur, for example, because of water spray or fuel. Adulterations of this kind can, however, occur especially at standstill of the engine. For example, fuel particles from the exhaust gas or from the ambient can diffuse into the reference atmosphere and there bind oxygen particles. In this case, the partial pressure of the reference oxygen drops which leads to the condition that the voltage of the measuring cell drops and even changes the sign in dependence upon the intensity of the contamination of the reference atmosphere with fuel particles. The probe signal made incorrect in this case increases the emission of unwanted exhaust-gas particles because of its influence on the mixture formation of the engine.